Auxiliary power steering mechanisms are so designed, in general, that the valve's characteristic curve (working pressure - operating power characteristic curve) has a flat, gradual ascent in a relatively large angle-of-rotation range of the rotary slide valve, and then rises steeply to a maximum value as the operating power increases further. So the driver of the vehicle must turn the steering wheel through a relatively large angle of rotation, when parking and maneuvering, for example, until a greater intensification of power is reached. The two rotary slide members elastically connected with each other by the torsion bar rotate against each other within this angle of rotation. Depending upon the cross section and length of the torsion bar, the driver must apply a more or less large amount of operating power until the rotary slide valve triggers the desired intensification of power. Such a valve characteristic curve, with an initially flat ascent through the angle of rotation, makes very good straight-line travel characteristics possible at higher speeds since the auxiliary power portion remains small and the response within the elastic area of the torsion bar permits a good steering touch. However, the fact that the operating power is relatively high in the slow-travel range is a disadvantage of the characteristic curve characteristic that is synchronized with an exact steering behavior at high speeds.
A rotary slide valve that accomplishes the controlling of pressure so that the opposing demands for lower manual operating power in the slow-travel range and high manual operating power at fast speeds can be satisfied is already known from EP 01 71 247 A2. This known rotary slide valve is a structural part of a rack-and-pinion steering mechanism in which the rotary slide member connected with the steering shaft is connected with the pinion engaging with the steering rack by the torsion bar. The valve sleeve surrounding the rotary member is likewise coupled with the pinion in the direction of rotation with a steering play, but guided axially displaceably on the rotary sleeve. One face of the valve sleeve rests against a spring in the housing while a surge chamber is attached to its other face. This surge chamber lies between the radial return drilled holes in the rotary slide member and the return connection leading to a container. An electro-mechanically activated adjustment throttle is installed in the return connection that controls the return cross section as a function of the speed of travel. In rapid travel, the adjustment throttle is wide open, so that the oil flows unhindered to the return connection. Therefore the valve sleeve remains on the rotary slide member in its initial position, determined by the spring, so that the guide grooves and the guiding edges of the rotary slide valve corresponding to them are available for regulating the pressure along their entire length. This provides a characteristic curve path with a gradual increase of pressure over a relatively large angle-of-rotation range and a further steep increase of pressure only toward the end of the relative rotation of the two parts of the rotary slide valve to each other.
At slow speed (i.e. when parking), the cross-sectional area of an aperture at the adjustment throttle is narrowed down sharply. As a result, a differential pressure builds up in the surge chamber of the valve sleeve that moves it into a final position against the spring resistance. The effective guiding-edge length of the rotary slide valve is shortened, so that the original characteristic curve of the rotary slide valve is changed. The torsion angle between the two rotary slide structural parts needed for regulating the pressure is smaller since the building up of pressure in the active pressure chamber of the working cylinder takes place much faster. This is a result of the reduced regulating cross section. Since the torsion bar consequently must only be rotated through a small angle, the operating power at the steering wheel is also correspondingly smaller. Now the steering power characteristic curve has a very steep path through the angle of rotation. Between the two described positions of the valve sleeve--that is, the initial position and the final position--any intermediate position of the valve sleeve at all is possible as a function of the variable differential pressure. In this way, the valve characteristic curve can be adapted to the driving situation of the moment. For better synchronization of the characteristic curve, the guiding edges of the known rotary slide are provided with short pockets that gradually change into long control slots. The peripheral length of the pockets is larger than the slots.
This known oil return control system for changing the valve characteristic curve is only suitable for steering devices with an axially movable valve sleeve. In steering gears in which such a valve sleeve is secured to another component for structural reasons, the return control system that has been described cannot be used. As a result of the great length of packing on their outer circumference, each axially displaceable valve sleeves are subject to operation.